1. Field of the Invention
The present invention relates to a photothermographic material, particularly to a photothermographic material suitable for medical imaging, industrial photographic imaging, graphic arts and COM.
2. Description of the Related Art
Recently, in the medical imaging field and the graphic arts field, dry photographic process is strongly desired from the standpoints of environmental conservation and space saving. In these fields, digitization has been in progress, and systems are quickly spreading in which image information is taken into a computer and preserved, and when necessary, processed, and output on a photosensitive material by a laser image setter or laser imager at a necessary position using communication, and developed. As the photosensitive material, photosensitive materials which can be recorded by exposure to laser of high intensity of illumination and can form a clear black image having high resolution and sharpness are required. As such digital recording imaging materials, various hard copying systems utilizing pigments and dyes such as inkjet printers, electrophotography and the like are distributed as a general image formation system, however, they are unsatisfactory in image qualities (sharpness, graininess, gradation, tone) determining a diagnosis ability such as in medical imaging, and in recording speed (sensitivity), and have not reached level capable of substituting conventional wet processing silver salt films for medical use.
On the other hand, thermal image formation systems utilizing an organic silver salt are described in U.S. Pat. Nos. 3,152,904, 3,457,075, and D. H. Klosterboer, Thermally Processed Silver Systems (Imaging Processes and Materials, Neblette, 8th edition, Sturge, V. Walworth and A. Shepp, edit, chapter 9, p. 279, 1989).
A photothermographic material is exposed image-wisely, then, heated at high temperature (for example, 80° C. or higher), and gives a black silver image formed by a redox reaction between a silver halide or reducible silver salt (functions as an oxidizer) and a reducing agent. The redox reaction is promoted by the catalytic action of a latent image generated on a silver halide by exposure. As a result, a black silver image is formed on the exposed region. The photothermographic materials are disclosed in a lot of literatures typically including U.S. Pat. No. 2,910,377 and JP-B No. 43-4924.
In the photothermographic materials as described above, polymers having glass transition temperatures in a range lower than the thermal development temperature are used as a binder.
On the other hand, generally used as laser beam are gas lasers (Ar+, He—Ne, He—Cd), YAG laser, dye laser, semiconductor laser and the like. Semiconductor laser and second harmonic generation element and the like can also be used. As mentioned to emitting wavelengt, there are used lasers in wider wavelength range from blue range to infrared range. Of them, infrared semiconductor laser is particularly suitable for designing of a image output system by laser, which is compact and excellent in operability and does not restrict the situation place and used conveniently since economic and stable in light emission is obtained. The photothermographic material is required to have infrared sensitivity for the above-mentioned reason. Various efforts have been conducted for enhancing infrared sensitivity. However, infrared spectrum sensitization has a problem that it is in general unstable and decomposed during the preservation of the photosensitive material, leading to decrease in sensitivity, and there is an increasing requirement for improvement in preservation stability, together with increased sensitivity.
Recently, blue semiconductor laser has been developed to enable image recording with high precision and consequently, recording density increases and long life and stable output is obtained, therefore, demand for the blue semiconductor laser is expanding and a photothermographic recording material corresponding to this is required.
Since the photothermographic material contains all chemicals necessary for image forming incorporated in the layers, the photothermographic material has a problem of preservability showing “increase in fogging” in which a non-exposed portion is blackened by preservation until use after production of the photothermographic material and a problem of “print out” in which a non-exposed portion is blackened gradually when an image is left under weak light such as room light and the like after photothermographic development.
As a means of improving this print out, incorporation of a halogen precursor compound and of other development termination agents and the like have been suggested, however, in any means, image formation itself is disturbed and sensitivity is lowered, resulting in insufficient effects.
Particularly, in the case of the photothermographic material by an organic solvent application method using polyvinyl butylal as a binder, there is a problem that variation in sensitivity during preservation is larger as compared to that by water application method using a polymer latex. Under such conditions, there is a desired for a technology of increasing sensitivity giving excellent preservation stability particularly when an organic solvent is used as the application solvent.
Thus, the print out and fogging is a very important problem in case of a photothermographic material, and improvement of these problems is always eagerly desired.
An application of silver iodide as a photosensitive silver halide has been tried, however, it has very low sensitivity and practical use thereof has not been take into consideration.
As a means of increasing the sensitivity of a silver iodide photographic emulsion, academic literatures disclose addition of a halogen receptor such as sodium nitrite, pyrogallol, hydroquinone and the like, immersion into an aqueous silver nitrate solution, sulfur sensitization at a pAg of 7.5, and the like.
For example, these are described in P. B. Gilman, Photographic Science and Engineering, 18(5), 475 (1974), W. L. Gardener, Photographic Science and Engineering, 21(6), 325 (1977), T. H. James, Photographic Science and Engineering, 5, 216 (1961), and the like.
However, the sensitization effect of these halogen acceptors is very small and extremely insufficient in a photothermographic material intended by the invention.
In a photothermographic material sensitized to infrared light, sensitivity is tried to be increased by using a heteroaromatic mercapto compound or heteroaromatic disulfide compound as a supersensitizer. When silver iodide is used as a photosensitive silver halide, these compounds have an action of increasing sensitivity, but also have problems that color tone of the image varies and pure black tone is not obtained easily, development is suppressed and a long period development time is necessary for image formation, and the like.